Born to run? Integrating individual behavior, physiology, and life histories in partially migratory steelhead and rainbow trout (Oncorhynchus mykiss)

Steelhead and rainbow trout are common names for marine-migratory (anadromous) and freshwater-resident forms of Oncorhynchus mykiss, a partially migratory salmonid fish. Anadromous and resident forms are sympatric and can produce offspring with a life history different from their own (i.e., steelhead parents can produce rainbow trout offspring and vice versa). The expression of these alternative life histories is a plastic response to individual patterns of energy acquisition, assimilation, and allocation during juvenile life stages. Individual performance during early stream life is of particular interest because of potential carry-over effects on subsequent growth and developmental trajectories. In a series of experiments in laboratory streams, I determined the influence of individual variation in energy metabolism on behavior, growth, and life-history expression in O. mykiss. Individual variation in energy metabolism was a strong predictor of feeding territory acquisition by juvenile fish during the transition from dependence on maternal provisioning of egg yolk reserves to independent feeding. Feeding territory acquisition was positively associated with standard metabolic rate (SMR) under conditions of an abundant and predictable food supply. When the density of intraspecific competitors was manipulated, the association between SMR and territory acquisition was strongest at intermediate stocking densities, moderate at the highest stocking densities, and weakest at the lowest stocking densities. However, reducing the spatial predictability of food resources within streams reversed the influence of SMR on competitive outcomes. These experiments determined that variation in ecological conditions during early life stages imposes different selection regimes on juvenile O. mykiss and results in physiological divergence among cohorts. Subsequent rearing experiments determined that behavioral dominance influences rates of anadromy and freshwater maturation, most likely through the association between SMR and territory acquisition. In addition to the effects of behavioral dominance, I observed a significant influence of sex, rearing temperature, and individual growth trajectories on the expression of anadromy and freshwater maturation. Partially migratory populations of O. mykiss maintain an exceptionally diverse portfolio of life-history strategies. Results from this work lend insight into a suite of behavioral and physiological processes influencing individual life histories

Demographic and phenotypic responses of juvenile steelhead trout to spatial predictability of food resources

We manipulated food inputs among patches within experimental streams to determine how variation in foraging behavior influenced demographic and phenotypic responses of juvenile steelhead trout (Oncorhynchus mykiss) to the spatial predictability of food resources. Demographic responses included compensatory adjustments in fish abundance, mean fish size, and size inequality. These responses paralleled shifts in individual foraging behavior, which increased the strength of exploitative competition relative to interference competition in streams with lower spatial predictability of food resources. Variation in the spatial predictability of food resources also favored different physiological phenotypes, as inferred from selection on an index of standard metabolic rate (SMR) based on fish otolith size. We observed positive directional selection on SMR in streams with spatially predictable food resources, disruptive selection for SMR at intermediate levels of spatial predictability, and negative directional selection for SMR in streams with the lowest level of spatial predictability of food resources. Thus, variation in the spatial predictability of food resources resulted in changes in individual behavior and modes of population regulation, and produced physiologically divergent cohorts of stream salmonids.Keywords: Emigration, Population dynamics, Spatial predictability, Phenotypic selection, Competition, Regulation, Oncorhynchus mykiss, Steelhead trout, Growth depensation, Intraspecific competition, Standard metabolic rate, Exploitation and interferenc

Estimating Habitat-Specific Abundances of PIT-Tagged Juvenile Salmonids Using Mobile Antennas: A Comparison with Standard Electrofishing Techniques in a Small Stream

The use of passive integrated transponder (PIT) tags in ecological studies is increasingly common, but evaluations of their performance and range of application in the field are still emerging. Here, we compare habitat unit–scale abundance estimates of PIT-tagged juvenile coho salmon Oncorhynchus kisutch and steelhead O. mykiss derived from multiple-pass surveys with a submersible pole-mounted PIT antenna and backpack electrofishing in a small coastal California stream. We found high concordance of methods for coho salmon and age-0 steelhead and moderate concordance for age-1 and older steelhead. Regression analysis of PIT antenna estimates on electrofishing estimates indicated an approximately one-to-one relationship between methods. Regression intercepts for coho salmon and age-0 steelhead were significantly greater than zero, but the differences were small (with an effect equivalent to less than one fish). We found no evidence that pool area, cover area, or cover complexity influenced the relationship between methods for coho salmon and age-0 steelhead, but some evidence that electrofishing may be more effective than PIT antennas for estimating the abundance of age-1 and older steelhead in pools with high cover complexity. Our results demonstrate that surveys using submersible PIT antennas can provide habitat unit–scale estimates of juvenile salmonid abundance similar to those derived from electrofishing in small streams. An advantage of using PIT antennas is that they allow fish abundance estimation without fish recapture and frequent sampling can occur without subjecting study animals to excessive handling stress or mortality. This is a particularly important consideration in studies of small populations, sensitive species, or fish listed under the U.S. Endangered Species Act.This is the publisher’s final pdf. The published article is copyrighted the American Fisheries Society and can be found at: http://www.tandfonline.com/toc/ujfm20/current

Influence of sex, migration distance, and latitude on life history expression in steelhead and rainbow trout (Oncorhynchus mykiss)

In partially migratory species, such as Oncorhynchus mykiss, the emergence of life history phenotypes is often attributed to fitness trade-offs associated with growth and survival. Fitness trade-offs can be linked to reproductive tactics that vary between the sexes, as well as the influence of environmental conditions. We found that O. mykiss outmigrants are more likely to be female in nine populations throughout western North America (grand mean 65% female), in support of the hypothesis that anadromy is more likely to benefit females. This bias was not related to migration distance or freshwater productivity, as indicated by latitude. Within one O. mykiss population we also measured the resident sex ratio and did not observe a male bias, despite a high female bias among outmigrants in that system. We provide a simulation to demonstrate the relationship between sex ratios and the proportion of anadromy and show how sex ratios could be a valuable tool for predicting the prevalence of life history types in a population

Ecological and evolutionary patterns of freshwater maturation in Pacific and Atlantic salmonines

Reproductive tactics and migratory strategies in Pacific and Atlantic salmonines are inextricably linked through the effects of migration (or lack thereof) on age and size at maturity. In this review, we focus on the ecological and evolutionary patterns of freshwater maturation in salmonines, a key process resulting in the diversification of their life histories. We demonstrate that the energetics of maturation and reproduction provides a unifying theme for understanding both the proximate and ultimate causes of variation in reproductive schedules among species, populations, and the sexes. We use probabilistic maturation reaction norms to illustrate how variation in individual condition, in terms of body size, growth rate, and lipid storage, influences the timing of maturation. This useful framework integrates both genetic and environmental contributions to conditional strategies for maturation and, in doing so, demonstrates how flexible life histories can be both heritable and subject to strong environmental influences. We review evidence that the propensity for freshwater maturation in partially anadromous species is predictable across environmental gradients at geographic and local spatial scales. We note that growth is commonly associated with the propensity for freshwater maturation, but that life-history responses to changes in growth caused by temperature may be strikingly different than changes caused by differences in food availability. We conclude by exploring how contemporary management actions can constrain or promote the diversity of maturation phenotypes in Pacific and Atlantic salmonines and caution against underestimating the role of freshwater maturing forms in maintaining the resiliency of these iconic species.Keywords: Maturation, Salmon, Diversity, Salmo, Oncorhynchus, Oncorhynchus Resilience, Life history, Conservatio

Anadromy and residency in steelhead and rainbow trout (Oncorhynchus mykiss): a review of the processes and patterns

Oncorhynchus mykiss form partially migratory populations with anadromous fish that undergo marine migrations and residents that complete their life cycle in fresh water. Many populations’ anadromous components are threatened or endangered, prompting interest in understanding ecological and evolutionary processes underlying anadromy and residency. In this paper, we synthesize information to better understand genetic and environmental influences on O. mykiss life histories, identify critical knowledge gaps, and suggest next steps. Anadromy and residency appear to reflect interactions among genetics, individual condition, and environmental influences. First, an increasing body of literature suggests that anadromous and resident individuals differ in the expression of genes related to growth, smoltification, and metabolism. Second, the literature supports the conditional strategy theory, where individuals adopt a life history pattern based on their conditional status relative to genetic thresholds along with ultimate effects of size and age at maturation and iteroparity. However, except for a generally positive association between residency and high lipid content plus a large attainable size in fresh water, the effects of body size and growth are inconsistent. Thus, individuals can exhibit plasticity in variable environments. Finally, patterns in anadromy and residency among and within populations suggested a wide range of possible environmental influences at different life stages, from freshwater temperature to marine survival. Although we document a number of interesting correlations, direct tests of mechanisms are scarce and little data exist on the extent of residency and anadromy. Consequently, we identified as many data gaps as conclusions, leaving ample room for future research

Glacier retreat creating new Pacific salmon habitat in western North America

Glacier retreat poses risks and benefits for species of cultural and economic importance. One example is Pacific salmon (Oncorhynchus spp.), supporting subsistence harvests, and commercial and recreational fisheries worth billions of dollars annually. Although decreases in summer streamflow and warming freshwater is reducing salmon habitat quality in parts of their range, glacier retreat is creating new streams and lakes that salmon can colonize. However, potential gains in future salmon habitat associated with glacier loss have yet to be quantified across the range of Pacific salmon. Here we project future gains in Pacific salmon freshwater habitat by linking a model of glacier mass change for 315 glaciers, forced by five different Global Climate Models, with a simple model of salmon stream habitat potential throughout the Pacific Mountain ranges of western North America. We project that by the year 2100 glacier retreat will create 6,146 (±1,619) km of new streams accessible for colonization by Pacific salmon, of which 1,930 (±569) km have the potential to be used for spawning and juvenile rearing, representing 0 to 27% gains within the 18 sub-regions we studied. These findings can inform proactive management and conservation of Pacific salmon in this era of rapid climate change.Ye

Vignette 23: Indigenous Management Systems Can Promote More Sustainable Salmon Fisheries in the Salish Sea

Indigenous peoples of the Northern Pacific Rim have harvested salmon for more than 10,000 years, and Pacific salmon (Oncorhynchus spp.) form the foundation of social-ecological systems encompassing communities from California to Kamchatka and Northern Japan. Through continuous placed-based interdependence with salmon, Indigenous societies formed deliberate and well-honed systems of salmon management. These systems promoted the sustained productivity of salmon fisheries. In Canada and the United States, Indigenous sovereignty and resource stewardship were forcibly disrupted by colonial government authority. Despite the destructive impacts of colonization, Indigenous culture and knowledge are resurgent in Canada and the United States. Indigenous fishing technologies and management systems are being documented and reinvigorated. Systems of Indigenous salmon management can support long- term opportunities for equitable and sustainable harvest of salmon across western North America

Appendix B. Schematic of stream mesocosms and experimental layout.

Schematic of stream mesocosms and experimental layout

Appendix A. Summary of logistic regression selection analyses.

Summary of logistic regression selection analyses